DE973261C - Solution to increase the corrosion resistance of aluminum and its alloys and processes for their application - Google Patents

Description

ISSUED DECEMBER 31, 1959

A 21949 VII48d

(V. St. A.)

The invention aims, the corrosion resistance of surfaces of aluminum and its To increase alloys significantly without significantly impairing the metallic appearance of the same.

There are numerous methods of increasing the corrosion resistance of aluminum and its Alloys known. These can be divided into electrochemical and purely chemical processes ίο be. The latter displace the former more and more, since those require expensive electrical equipment, have lower throughput, and thus are considerably more expensive to install and operate. On the other hand, the electrochemical processes have the advantage that they make it possible to produce very thin, uniform oxide layers, which make the metallic Do not affect the appearance of the surface, or only very little. This was previously the case with the purely chemical Procedure not possible; Rather, these always caused a discoloration of the surface, mostly in the form of different colored spots.

The invention enables such a metallic surface, which is often desired, also in a purely chemical way without getting discoloration and stains that

909 681/4

corresponds in appearance and corrosion resistance to the electrochemically generated surfaces.

Among the numerous solutions already proposed for the creation of protective layers on aluminum there are also those containing chromic acid or chromates and double fluorides with or without other additives such as phosphoric acid, arsenic acid, halogens, boron compounds, etc.

ίο It has now been found that comparatively much dilute solutions of chromic acid or chromates and double fluorides in a peculiar way on the Act on the surface of aluminum and its alloys. If you let such a solution on a The aluminum surface, which has been freed from the always present oxide layer, has an effect, as this shows a relatively low electrical surface resistance immediately after exposure, which increases very quickly and after a short time Time far exceeds that of an untreated surface. A maximum of the surface resistance is reached in about 24 hours. Accordingly, the treated according to the invention withstands Surface without a visible protective layer to the salt spray corrosion test (e.g. USA. Government Specification Q-Q-M-151 / a) at least 168 hours, while the same, previously freed from the oxide layer, but untreated surface already after Shows severe corrosion for 24 hours. This behavior can perhaps be explained by this are that the treatment according to the invention activates the surface «so that they can be in the shortest possible time the oxygen in the air creates an even, very thin, invisible and uninterrupted layer of oxide forms.

Due to the use of very dilute solutions, due to the relatively great insensitivity against the time and temperature of the treatment and through the elimination of the requirement, To remove the oxide film that is always present on aluminum surfaces beforehand, the method according to the invention shows itself Process economically and technically superior to known processes.

In a known process with very wide concentration ranges of chromate and fluorine, primarily visible and colored layers are to be produced. It is also mentioned that colorless layers can be obtained. However, this is not achieved by reducing the amounts of chromic acid and hydrofluoric acid, but by adding larger amounts of potassium alum. Even if these colorless layers of their lack of color and corrosion protection layers according to the invention would correspond in such sets the new process because of the economic efficiency of the solutions of the invention, which is only about ^χ / ₁₀ on chromic acid and HF and no Alaunzusatz contain a erheblicnen technical Fortschri + t is .

According to the invention solutions are used as active ingredients 0.4 to 4 g / l of CrO ₃ and 0.1 contain up ι g / l F and having a p _H 2 to 4 The effect drops sharply below the lower limit for CrO _{3 and F;} above the upper limit there is a tendency to form visible and colored coatings. At a p _H less than 2, the tendency to form a visible coating, no significant action is carried out at a p _H 4 onto the surface. The adjustment of the p _H -value in a known manner either by the use of chromic acid and / or hydrogen fluoride complex Saur s in the preparation of the solutions or concentrates or by adding other acids, or by blunting to acidic solutions with alkali.

The invention is illustrated by the following examples:

CrO ₃

K ₂ ZrF ₆

K ₂ TiF ₆ H ₂ O

Water ad

example

II

0.46 g 0.29 g

ι 1

2.0 g

4 g ι 1

The surfaces to be treated must of course be free of dirt and oil, so that they can let the treatment solution wet evenly; on the other hand, he always needs on such surfaces existing light oxide film not to be removed, but it still increases the passivation and the corrosion resistance when the surface is treated according to the invention.

The solution of Example I is particularly suitable for the immersion process. The duration of the dive can be a bath temperature of about 70 ° be about 1 to 2 minutes, after which it is rinsed and dried.

The solution of Example II is particularly suitable for application with a brush at room temperature, whereupon let them act for 1 to 2 minutes. The solution can then be allowed to dry up, but a powdery residue easily remains on the metal surface. Although this the corrosion resistance not affected, a water rinse is recommended if there is such a residue is undesirable.

As said above, the time and temperature of the treatment are not very critical. The temperature can be from 20 ⁰ to the boiling point of the solution. It is advisable to use more concentrated solutions (within the specified limits) at low temperatures than at high temperatures, the treatment time being chosen so that no visible coating or discoloration of the surface is noticeable.

Instead of the above-mentioned complex fluorides of zirconium and titanium, those of silicic acid, boric acid, tin, etc. can be used. iao

Instead of dipping or applying a brush, the solution can of course also be applied by spraying will. The object can also be removed from the solution immediately after immersion and allow the adhering film to act for 1 to 2 minutes.

As the above examples show, the _{more F the solution contains, the more CrO 4} it contains, and vice versa.

Claims (3)

Patent claims: i. Solution for the treatment of aluminum and its alloys for the purpose of increasing the corrosion resistance, which contains hexavalent chromium and a double fluoride as effective components, characterized in that the CrO ₃ content of the solution is 0.4 to 4 g / l and the F content is 0, 1 to ig / 1 and the _pH value is 2 to 4. 2. A method of using a solution according to claim i, characterized in that treating the metal surface without removing any existing on the surface Oxide layer takes place. 3. The method according to claim 2, characterized in that the treatment at room temperature he follows. ® 609 529/516 5.56 (909 681/4 12.59)